One method of producing and transferring monomolecular layers (Langmuir-Blodgett method) is described by Langmuir (J. Am. Chem. Soc. 57 (1935) 1007-1010). In the conventional method for producing these layers, amphiphilic molecules which are not water-soluble are spread in the form of a solution on a water surface of a so-called Langmuir trough. After the solvent has evaporated, the molecules are compressed to an unbroken film by reducing the water surface area available to them until the compression state is suitable for transferring the film. The reduction in surface area is effected by moving a solid barrier or else by displacing a one-piece flexible band which encloses the amphiphilic molecules on the water surface. Transfer of the film from the water surface to a substrate is effected by immersing or withdrawing the substrate through the film-covered water surface. In particular, by repeating the dipping processes it is possible to transfer multilayers. Such multilayers are currently of great interest for various applications, for example in sensorics, optics and electronics.
However, when relatively large substrates are coated with multilayers, the conventional method for producing the layers is very complicated. Owing to the fact that the film available on the water surface is used up after a few film cycles, dipping production by means of the above-described processes of fill spreading, solvent evaporation and compression of the film have to be repeated in order to enable coating of the substrate to be continued.
The literature describes a number of methods which, in order to eliminate this problem, are directed towards a continuous production of the film on the water surface. Barraud et al. (Thin Solid Films 99 (1983) 221) describe a continuous technique in which the film is compressed by means of rotating rollers. The disadvantage of this method is that the structure of the compressed fill is impaired by the shearing movements near the rollers, especially near the edges thereof.
U.S. Pat. No. 4,722,856 describes a process in which the spread molecules are compressed by causing the water to flow from a higher zone of the trough via an incline into a lower zone. As the water flows down the slope, the amphiphilic molecules flow along with the water on the water surface and are dammed up in the lower trough zone to give a compressed film. Shearing movements in the film at the edges of the incline may give rise to an impairment of film quality. This is the reason why in one trough type the lower zone is designed in the form of a circle in order to ensure that the incline surrounds the entire lower zone, so that no edges are present.
A further process for the continuous production of compressed films on a water surface is described in European Application 0,433,325. In this process, the film is compressed at a horizontally arranged weir by having the water flow in a duct in order to decrease the cross-section and to increase the flow rate, the duct may exhibit a ramp at the bottom. The force necessary for compressing the film is exerted on the film as frictional force of the water, the film covering the liquid of the flow duct. The degree of film compression power is dictated by the shape and length of the duct, by the amount of water flowing through it per unit time and by the water level in the compression zone of the duct, which in turn is influenced by the flow rate. This interaction makes it difficult to adjust and control the compression state.
Various amphiphilic materials show, as a film on the water surface, a pronounced aging behavior, such as is, for example, the case with 22-tricosenoic acid. This may have the effect that optimum transfer of the film to substrates is only possible in certain aging states. In conventional batchwise processes, the residence time of a film once obtained from spreading or compression can only be set to a specific value prior to transfer of the first layer, since, as soon as the first layer is transferred, the compressed film undergoes additional aging during the time necessary for transferring the layers. In contrast, in continuous processes, the film residence time, after a running-in phase, remains constant from compression to transfer, which enables transfer of a uniformly aged film. However, in the known methods, the residence time is largely fixed by reasons of design and hardly adjustable. One way of adjusting it is given in those cases where, in addition to the transfer process, film material is removed from the water surface in a different manner, for example by suction. However, this is disadvantageous because it results in increased consumption of material and entails additional control requirements.
Certain applications of Langmuir-Blodgett layers, for example in non-linear optics, or pyroelectrically active layers require hetero multilayer systems or alternating layer systems. These are those multilayers in which successive immersion/withdrawal processes transfer different layer materials; for example, upon immersion of the substrate, a film of type A and, upon withdrawal, a film of type B are transferred, and by repeating this sequence alternating multilayer systems are built up. Processes enabling such layers to be produced are described, for example, in EP 0,183,426 A2 and in GB 2,165,471 A.
These processes for producing alternating layer systems operate according to the conventional principle of film compression by means of barriers. In this method, two devices for compressing two films in a Langmuir trough are combined in such a manner that a joint water phase is present and the substrate to be coated is immersed in the water through one film, transported by means of a suitable mechanical transporting device below the water surface to a position below the other film and withdrawn there again through the other film. Above the water surface, the substrate is conveyed to the starting point, and the coating cycle can start again. GB 2,165,471 too already describes an arrangement in which two chambers containing a subphase on whose surface monomolecular layers of a first and second amphiphilic compound were produced are separated by a wall which separates the subphases at least on the surface of the liquid and contains a sluice for a substrate.
The known processes for producing alternating multilayer systems also have the disadvantage, as described above, that with increasing size of the substrate surfaces to be coated layer production becomes more and more complicated due to limitations of the available film areas on the water. On the other hand, the known continuous processes for layer production are not suitable for alternating multilayers, in particular not for producing high-quality layers.